Fix build on FreeBSD 11-CURRENT, as described in issue #413.

This caused by a combination of the iperf3 build somehow using
the system queue.h on FreeBSD 11 (possibly only on this platform)
and TAILQ_END not being defined in the system queue.h.

Expanding the TAILQ_END macro to NULL seems to solve the problem.

Submitted by:	@rbgarga

src/iperf_api.c

@@ -2601,7 +2601,7 @@ iperf_free_stream(struct iperf_stream *sp)
     close(sp->buffer_fd);
     if (sp->diskfile_fd >= 0)
 	close(sp->diskfile_fd);
-    for (irp = TAILQ_FIRST(&sp->result->interval_results); irp != TAILQ_END(sp->result->interval_results); irp = nirp) {
+    for (irp = TAILQ_FIRST(&sp->result->interval_results); irp != NULL; irp = nirp) {
         nirp = TAILQ_NEXT(irp, irlistentries);
         free(irp);
     }